diff --git a/coq/_CoqProject b/coq/_CoqProject
index cf9c445..5841128 100644
--- a/coq/_CoqProject
+++ b/coq/_CoqProject
@@ -1,5 +1,6 @@
 -R . LadderTypes
 terms.v
+terms_debruijn.v
 equiv.v
 subst.v
 subtype.v
diff --git a/coq/terms_debruijn.v b/coq/terms_debruijn.v
new file mode 100644
index 0000000..81d4181
--- /dev/null
+++ b/coq/terms_debruijn.v
@@ -0,0 +1,281 @@
+From Coq Require Import Strings.String.
+From Coq Require Import Lists.List.
+Import ListNotations.
+
+Require Import terms.
+
+Inductive type_DeBruijn : Type :=
+  | ty_id : string -> type_DeBruijn
+  | ty_fvar : string -> type_DeBruijn
+  | ty_bvar : nat -> type_DeBruijn
+  | ty_univ : type_DeBruijn -> type_DeBruijn
+  | ty_spec : type_DeBruijn -> type_DeBruijn -> type_DeBruijn
+  | ty_func  : type_DeBruijn -> type_DeBruijn -> type_DeBruijn
+  | ty_morph : type_DeBruijn -> type_DeBruijn -> type_DeBruijn
+  | ty_ladder : type_DeBruijn -> type_DeBruijn -> type_DeBruijn
+.
+
+Inductive expr_DeBruijn : Type :=
+  | ex_var : nat -> expr_DeBruijn
+  | ex_ty_abs : expr_DeBruijn -> expr_DeBruijn
+  | ex_ty_app : expr_DeBruijn -> type_DeBruijn -> expr_DeBruijn
+  | ex_abs   : type_DeBruijn -> expr_DeBruijn -> expr_DeBruijn
+  | ex_morph : type_DeBruijn -> expr_DeBruijn -> expr_DeBruijn
+  | ex_app : expr_DeBruijn -> expr_DeBruijn -> expr_DeBruijn
+  | varlet : type_DeBruijn -> expr_DeBruijn -> expr_DeBruijn -> expr_DeBruijn
+  | ex_ascend : type_DeBruijn -> expr_DeBruijn -> expr_DeBruijn
+  | ex_descend : type_DeBruijn -> expr_DeBruijn -> expr_DeBruijn
+.
+
+(* get the list of all free variables in a type term *)
+Fixpoint type_fv (τ : type_DeBruijn) {struct τ} : (list string) :=
+  match τ with
+  | ty_id s => []
+  | ty_fvar α => [α]
+  | ty_bvar x => []
+  | ty_univ τ => (type_fv τ)
+  | ty_spec σ τ => (type_fv σ) ++ (type_fv τ)
+  | ty_func σ τ => (type_fv σ) ++ (type_fv τ)
+  | ty_morph σ τ => (type_fv σ) ++ (type_fv τ)
+  | ty_ladder σ τ => (type_fv σ) ++ (type_fv τ)
+  end.
+
+(* substitute free variable x with type σ in τ *)
+Fixpoint subst_type (x:string) (σ:type_DeBruijn) (τ:type_DeBruijn) {struct τ} : type_DeBruijn :=
+  match τ with
+  | ty_id s => ty_id s
+  | ty_fvar s => if eqb x s then σ else τ
+  | ty_bvar y => ty_bvar y
+  | ty_univ τ => ty_univ (subst_type x σ τ)
+  | ty_spec τ1 τ2 => ty_spec (subst_type x σ τ1) (subst_type x σ τ2)
+  | ty_func τ1 τ2 => ty_func (subst_type x σ τ1) (subst_type x σ τ2)
+  | ty_morph τ1 τ2 => ty_morph (subst_type x σ τ1) (subst_type x σ τ2)
+  | ty_ladder τ1 τ2 => ty_ladder (subst_type x σ τ1) (subst_type x σ τ2)
+  end.
+
+(* replace a free variable with a new (dangling) bound variable *)
+Fixpoint type_bind_fvar (x:string) (n:nat) (τ:type_DeBruijn) {struct τ} : type_DeBruijn :=
+  match τ with
+  | ty_id s => ty_id s
+  | ty_fvar s => if eqb x s then ty_bvar n else τ
+  | ty_bvar n => ty_bvar n
+  | ty_univ τ1 => ty_univ (type_bind_fvar x (S n) τ1)
+  | ty_spec τ1 τ2 => ty_spec (type_bind_fvar x n τ1) (type_bind_fvar x n τ2)
+  | ty_func τ1 τ2 => ty_func (type_bind_fvar x n τ1) (type_bind_fvar x n τ2)
+  | ty_morph τ1 τ2 => ty_morph (type_bind_fvar x n τ1) (type_bind_fvar x n τ2)
+  | ty_ladder τ1 τ2 => ty_ladder (type_bind_fvar x n τ1) (type_bind_fvar x n τ2)
+  end.
+
+(* replace (dangling) index with another type *)
+Fixpoint type_open_rec (k:nat) (σ:type_DeBruijn) (τ:type_DeBruijn) {struct τ} : type_DeBruijn :=
+  match τ with
+    | ty_id s => ty_id s
+    | ty_fvar s => ty_fvar s
+    | ty_bvar i => if Nat.eqb k i then σ else τ
+    | ty_univ τ1 => ty_univ (type_open_rec (S k) σ τ1)
+    | ty_spec τ1 τ2 => ty_spec (type_open_rec k σ τ1) (type_open_rec k σ τ2)
+    | ty_func τ1 τ2 => ty_func (type_open_rec k σ τ1) (type_open_rec k σ τ2)
+    | ty_morph τ1 τ2 => ty_morph (type_open_rec k σ τ1) (type_open_rec k σ τ2)
+    | ty_ladder τ1 τ2 => ty_ladder (type_open_rec k σ τ1) (type_open_rec k σ τ2)
+  end.
+
+Notation "'[' z '~>' u ']' e" := (subst_type z u e) (at level 68).
+Notation "'{' k '~>' σ '}' τ" := (type_open_rec k σ τ) (at level 67).
+Definition type_open σ τ := type_open_rec 0 σ τ.
+
+(* is the type locally closed ? *)
+Inductive type_lc : type_DeBruijn -> Prop :=
+  | Tlc_Id : forall s, type_lc (ty_id s)
+  | Tlc_Var : forall s, type_lc (ty_fvar s)
+  | Tlc_Univ : forall τ1 L,
+      (forall x, ~ (In x L) -> type_lc (type_open (ty_fvar x) τ1)) ->
+      type_lc (ty_univ τ1)
+  | Tlc_Spec : forall τ1 τ2, type_lc τ1 -> type_lc τ2 -> type_lc (ty_spec τ1 τ2)
+  | Tlc_Func : forall τ1 τ2, type_lc τ1 -> type_lc τ2 -> type_lc (ty_func τ1 τ2)
+  | Tlc_Morph : forall τ1 τ2, type_lc τ1 -> type_lc τ2 -> type_lc (ty_morph τ1 τ2)
+  | Tlc_Ladder : forall τ1 τ2, type_lc τ1 -> type_lc τ2 -> type_lc (ty_ladder τ1 τ2)
+.
+
+(* number of abstractions *)
+Fixpoint type_debruijn_depth (τ:type_DeBruijn) : nat :=
+  match τ with
+  | ty_id s => 0
+  | ty_fvar s => 0
+  | ty_bvar x => 0
+  | ty_func s t => max (type_debruijn_depth s) (type_debruijn_depth t)
+  | ty_morph s t => max (type_debruijn_depth s) (type_debruijn_depth t)
+  | ty_univ t => (1 + (type_debruijn_depth t))
+  | ty_spec s t => ((type_debruijn_depth s) - 1)
+  | ty_ladder s t => max (type_debruijn_depth s) (type_debruijn_depth t)
+  end.
+
+Fixpoint type_named2debruijn (τ:type_term) {struct τ} : type_DeBruijn :=
+  match τ with
+  | type_id s => ty_id s
+  | type_var s => ty_fvar s
+  | type_univ x t => let t':=(type_named2debruijn t) in (ty_univ (type_bind_fvar x 0 t'))
+  | type_spec s t => ty_spec (type_named2debruijn s) (type_named2debruijn t)
+  | type_fun s t => ty_func (type_named2debruijn s) (type_named2debruijn t)
+  | type_morph s t => ty_morph (type_named2debruijn s) (type_named2debruijn t)
+  | type_ladder s t => ty_ladder (type_named2debruijn s) (type_named2debruijn t)
+  end.
+
+Coercion type_named2debruijn : type_term >-> type_DeBruijn.
+
+Lemma list_in_tail : forall x (E:list string) f,
+  In x E ->
+  In x (cons f E).
+Proof.
+  intros.
+  simpl.
+  right.
+  apply H.
+Qed.
+
+Lemma list_in_concatA : forall x (E:list string) (F:list string),
+  In x E ->
+  In x (E ++ F).
+Proof.
+Admitted.
+
+Lemma list_in_concatB : forall x (E:list string) (F:list string),
+  In x F ->
+  In x (E ++ F).
+Proof.
+  intros.
+  induction E.
+  auto.
+  apply list_in_tail.
+  
+Admitted.
+
+Lemma list_notin_singleton : forall (x:string) (y:string),
+  ((eqb x y) = false) -> ~ In x [ y ].
+Proof.
+Admitted.
+
+Lemma list_elim_notin_singleton : forall (x:string) (y:string),
+  ~ In x [y] -> ((eqb x y) = false).
+Proof.
+Admitted.
+
+
+Lemma subst_fresh_type : forall (x : string) (τ:type_DeBruijn) (σ:type_DeBruijn),
+  ~(In x (type_fv τ)) ->
+  (subst_type x σ τ) = τ
+.
+Proof.
+  intros.
+  induction τ.
+
+  - reflexivity.
+
+  - unfold type_fv in H.
+    apply list_elim_notin_singleton in H.
+    simpl.
+    case_eq (x =? s)%string.
+    congruence.
+    reflexivity.
+
+  - reflexivity.
+
+  - simpl. rewrite IHτ.
+    reflexivity.
+    apply H.
+
+  - simpl. rewrite IHτ1, IHτ2.
+    reflexivity.
+    simpl type_fv in H.
+    contradict H. apply list_in_concatB, H.
+    contradict H. apply list_in_concatA, H.
+
+  - simpl. rewrite IHτ1, IHτ2.
+    reflexivity.
+    contradict H. apply list_in_concatB, H.
+    contradict H. apply list_in_concatA, H.
+
+  - simpl. rewrite IHτ1, IHτ2.
+    reflexivity.
+    contradict H. apply list_in_concatB, H.
+    contradict H. apply list_in_concatA, H.
+
+  - simpl. rewrite IHτ1, IHτ2.
+    reflexivity.
+    contradict H. apply list_in_concatB, H.
+    contradict H. apply list_in_concatA, H.
+Qed.
+
+
+
+Lemma open_rec_lc_core : forall τ j σ1 i σ2,
+  i <> j ->
+  {j ~> σ1} τ = {i ~> σ2} ({j ~> σ1} τ) ->
+  τ = {i ~> σ1} τ.
+
+Proof with (eauto with *).
+  induction τ;
+  intros j v i u Neq H;
+  simpl in *; try solve [inversion H; f_equal; eauto].
+
+(*  case (ty_bvar).*)
+    destruct (Nat.eqb j n)...
+    destruct (Nat.eqb i n)...
+  
+Admitted.
+
+
+Lemma type_open_rec_lc : forall k σ τ,
+  type_lc τ ->
+  { k ~> σ } τ = τ.
+Proof.
+  intros.
+  generalize dependent k.
+  induction H.
+  - auto.
+  - auto.
+  - intro k.
+    unfold type_open in *.
+    (*
+    pick fresh x for L.
+    apply open_rec_lc_core with (i := S k) (j := 0) (u := u) (v := x). auto. auto.
+    *)
+    admit.
+  - simpl. intro. rewrite IHtype_lc1. rewrite IHtype_lc2. reflexivity.
+  - simpl. intro. rewrite IHtype_lc1. rewrite IHtype_lc2. reflexivity.
+  - simpl. intro. rewrite IHtype_lc1. rewrite IHtype_lc2. reflexivity.
+  - simpl. intro. rewrite IHtype_lc1. rewrite IHtype_lc2. reflexivity.
+Admitted.
+
+Lemma type_subst_open_rec : forall τ1 τ2 σ x k,
+  type_lc σ ->
+  [x ~> σ] ({k ~> τ2} τ1) = {k ~> [x ~> σ] τ2} ([x ~> σ] τ1).
+Proof.
+  intros.
+  induction τ1.
+
+  (* id *)
+  - auto.
+
+  (* free var *)
+  - simpl.
+    case_eq (eqb x s).
+    intro.
+    apply eq_sym.
+    apply type_open_rec_lc, H.
+    auto.
+
+  (* bound var *)
+  - simpl.
+    case_eq (Nat.eqb k n).
+    auto.
+    auto.
+
+  (* univ *)
+  - simpl.
+    admit.
+
+  - simpl. rewrite IHτ1_1. rewrite IHτ1_2. reflexivity.
+  - simpl. rewrite IHτ1_1. rewrite IHτ1_2. reflexivity.
+  - simpl. rewrite IHτ1_1. rewrite IHτ1_2. reflexivity.
+  - simpl. rewrite IHτ1_1. rewrite IHτ1_2. reflexivity.
+Admitted.